This invention relates to continuous casting apparatuses and methods.
Continuous casting of metals and metal alloys of various kinds, both ferrous and nonferrous, has been undertaken for many years. The majority of the prior art discloses machines in which casting is performed by discharging molten metal between a pair of rollers which are continually cooled. It is possible to cast vertically downward, downward at an angle, or horizontally.
Continuous casting of metals is undertaken by two common methods that are similar in some respects. Briefly, continuous casting is performed by means of endless members e.g. mold blocks mounted on or forming continuous chains, or endless belts with moving side dams disposed between the belts. The endless members which are typically disposed horizontally or slanted at a small angle from the horizontal serve as the mold for the cast metal, e.g., billet, slab, sheet, plate, or strip. The endless members, moving in non-circular paths, come together tangentially in a casting region to form a casting mold channel and stay together long enough so that the metal is solidified enough to support itself after which the endless members separate and are carried back to the beginning of the casting region. This method of casting has proved efficient and economical particularly in the casting of shapes such as slab, plate or strip, which may be used as the finished product, or if desired, the shape may be subjected to reduction rolling as it emerges from the horizontally disposed casting machine.
As stated, these generally horizontally disposed continuous casting machines are predominantly of two types. The first type utilizes a pair of continuous belts which approach each other tangentially to form a movable mold therebetween. As the molten metal is introduced between the belts, the belt is cooled. The cooling is, however, somewhat inefficient, and the thickness of the strip varies because of the lack of stiffness in the belt. To prevent variations in the thickness and shape of the strip, the molten metal must be supplied to the mold at a low pressure which effects the casting process and causes surface and shape problems as well as deficiencies in the metal structure.
To overcome the inefficiencies in cooling, thickness, and cast metal quality control, the belt is replaced with a continuous chain in the second type of caster which has consecutive mold blocks attached to or actually forming the chain. The mold blocks provide a structure which can be externally cooled, internally cooled, or both externally and internally cooled. This structure efficiently cools the metal being molded between the caster blocks, and the continuous caster utilizing the mold blocks also provides increased stiffness which results in a uniform thickness of the strip. This process is, however, subject to other deficiencies. Where the consecutive mold blocks abut each other, molten metal can flow in between the blocks and solidify there creating protrusions extending from the molded metal across its width. These protrusions are commonly referred to as fins. The presence of fins on the molded strip interferes with the subsequent formation processes, such as rolling, to which the molded metal might be subjected.
Further, it is frequently necessary, during the casting of flat products such as sheets or strips, to adjust the width of the strip. To adjust the width of the strip, different widths of chains must be kept in stock or continuous, expensive, adjustable width side dams which are movable across the width of the blocks must be provided. Because of the weight and bulk of the chain, the change is a difficult, time consuming, and extremely costly procedure.
It has also been difficult to obtain high accuracies of strip thickness/shape with the continuous casting machines. As the molten metal moves along the length of the chain caster, the metal cools and solidifies in the mold channel. As the metal cools, the volume decreases thus changing the casting pressure applied to the metal as it solidifies in the mold channel. The metal may even lose contact with the mold channel. This slows cooling thus requiring a longer mold channel, and under some circumstances, this can lead to undesirable variations in thickness and other shape deformations. More frequently, this has adverse effects on the microstructure of the cast product.
Thus, the production of continuous cast products without fins is desirable to enhance the products fabricated from continuous casting process and increase the ability to subject the continuously cast metal to further processing. It is also desirable to change the mold width of a continuous caster utilizing a chain without changing the chain. Further, it is desirable to maintain the casting pressure on the metal as it solidifies. The production of continuous cast products without fins, shortening the stop time of a width change, changing the mold width without changing the chain, and controlling the casting pressure, translate directly into increased use of continuously cast products and a reduction of manufacturing expenses for continuously cast products.
There is, therefore, provided in the practice of this invention a novel continuous caster comprising a headbox and a mold channel defined between two endless chain assemblies. The headbox is positioned at an opening of the mold channel, and molten metal is fed through the headbox to the mold channel. Each chain of the two endless chain assemblies has a protrusion at an opposite side of the chains defining a width and depth of the mold channel. At least one of the endless chain assemblies is movable relative to the other chain assembly, so that the width of the mold channel can be adjusted.
In a preferred embodiment, both of the chain assemblies are movable with respect to each other, so that the metal being cast is maintained centrally in the chain caster when the width of the mold channel is adjusted. In the preferred embodiment, the caster further comprises two endless belt assemblies which correspond to the chain assemblies. Each belt assembly operates externally from the corresponding chain assembly to create a smooth mold channel which produces a casted product without fins. The belts can have the same width as the mold channel which requires the casting process to be stopped so that the belts can be changed and the width of the mold channel changed. The relatively light and easily removable belts can be changed in a substantially shorter period of time than the chains. The belts can also have a width greater than the width of the mold channel to adjust the width of the mold channel without changing the belt.
The invention is further directed to a novel continuous caster comprising first and second mold assemblies having first and second moving chains and belts moving in first and second closed chain and belt paths, respectively. The chain paths are internal relative to the belt paths and the corresponding belt and chain paths join over at least the part of their paths where the first and second paths pass in close proximity to define a mold channel. Because the belt operates externally from the chain, the smooth belt defines the surface of the mold channel and prevents finning. A headbox and tip are provided at the opening of the mold channel to supply molten metal to the mold channel.
In a preferred embodiment, the caster further comprises a tensioning mechanism attached to the belts whereby the belts are tightened and held tightly against the chain. The belts are preferably coated with a heat resistant material which acts as a mold release, non-wetting agent, and heat transfer moderator. Further, cooling systems are provided for each mold assembly. Each cooling system is associated with both the belt and chain of the respective mold assembly thereby reducing the amount of cooling required.
The invention is still further directed to a novel continuous caster comprising a plurality of mold assemblies. At least one of the mold assemblies comprises an endless chain having a plurality of mold blocks, an upstream drive pulley, and a downstream drag pulley. The drive pulley pushes the chain into the casting region and the drag pulley tends to prevent the chain from leaving the casting region. Thus, the chain is compressed in the casting region, and the mold blocks are pushed together so that there are no gaps between the mold blocks. Preferably two mold assemblies utilize this feature, and the drive coupled to the upstream pulley supplies at least 4 kW more power than the drag drive for a strip 1000 mm wide and 25 mm thick. The mold blocks in this embodiment preferably have interlocking tongue-in-groove features to prevent xe2x80x9croof tiling.xe2x80x9d
In another embodiment, the invention is directed to a continuous caster comprising a headbox, a tip, and two opposing mold assemblies defining a mold channel therebetween. The headbox is positioned at an opening of the mold channel and molten metal is fed to the mold channel through the headbox and tip. The molten metal flows through the length of the mold channel to an exit. A means for adjusting the depth of the mold channel along the length of the mold channel is provided so that a depth of the mold channel at the exit can be changed relative to a depth of the mold channel at the opening during operation of the caster. To allow the depth adjustment without stopping the casting operation, mold blocks of the mold assemblies define at least one slot located near an end of the block. A leg is slidably received in the slot, and a biasing member is interposed between a base of the slot and the leg to bias the leg against an opposing surface.
In a preferred embodiment, each mold assembly comprises mold blocks defining slots with legs slidably received in the slots, and biasing members interposed between the legs and the bases of the slots. In this arrangement the slots of each mold assembly are on the same side opposite the slots of the other mold assembly. The mold blocks are also provided with back up extensions adjacent to the slots and located outside the legs. The back up extensions engage the legs and support them against the outward pressure of the metal inside the mold channel.
The invention is still further directed to a novel method for changing the width of a cast product being cast in a continuous casting process on a chain caster having two mold assemblies forming a mold channel therebetween. An alloy is continuously melted and introduced into the mold channel with a headbox through a tip. The width of the cast product is adjusted by sliding at least one of the mold assemblies relative to the other in a direction substantially transverse to the direction of travel of the metal through the mold channel. In a preferred embodiment, the width of the mold channel is adjusted by sliding both mold assemblies equal distances relative to each other in opposite directions which are substantially transverse to the direction of travel of the metal alloy, so that the alloy remains centered in the chain caster. Further, belts are used to define at least a portion of the mold channel. If the width of the belts is the same as the mold channel, the casting operation must be temporarily stopped and the belts and tips changed in order to adjust the width of the cast product. If the width of the belt is greater than the mold channel, the width of the cast product may be adjusted by temporarily stopping the process and changing the tip only.
The invention is still further directed to a novel method for continuous casting of products without fins on a chain caster having two belt and chain assemblies forming a mold channel therebetween. The method comprises melting a metal alloy, and introducing the metal into the mold channel. Endless belts are translated through closed paths, and endless chains are translated through closed paths inside the belt paths. In a preferred embodiment, the method further comprises tensioning the belts to insure that the belts do not separate from the chains in the casting region.
Another novel method is provided according to the present invention for compensating for volumetric changes of a metal alloy to prevent undesirable deformation, abnormalities in the microstructure, and enhance cooling as the metal alloy shrinks from cooling during a continuous casting process on a chain caster having upper and lower mold assemblies defining a mold channel therebetween. The volumetric changes are compensated for by adjusting the depth of the mold channel throughout its length. This is accomplished by pressing a plurality of slidable upper and lower legs held in slots of the mold blocks against opposing mold blocks of the other assembly. The legs of the upper assembly are on opposite sides of the lower assembly. This is further accomplished by tilting one of the mold assemblies relative to the other to adjust the depth of the mold channel. Preferably, one of the mold assemblies is tilted relative to the other mold assembly to decrease the depth of the mold channel at the exit thereby compressing the resilient members near the exit of the chain caster.